The first example I gave, Portugal Laurel, grows like a weed in our Pacific Northwest. Seems to thrive in the Marine West-Coast conditions. Probably does well in the northern half of Mediterranean climate, too. But, I believe the canker issue presents itself when unusual heat stresses the species. The kind of heat so rare in coastal Oregon, Washington, and British Columbia. In those places, the canker can be kept at bay. Hence, the reason Portugal Laurel is very rare here (and it is exceedingly rare here) has nothing to do with cold hardiness but with other factors.

The stuff Mike wrote about the Yucca Rostrata is all appropriate. And I have certainly thought of those issues. However, just across the Delaware River in the coastal plain of Southern New Jersey, Rostrata -- if not abundant, is still seen in fairly good quantities. And most looking quite healthy. In Rostrata's case in this region, the main factor seems to be soil character. My soil is heavy clay. In New Jersey, fast-draining sand. And that's a huge difference. The Jersey soil profile also promotes successful cultivation of other Yucca species beside Rostrata.

Can Rostrata be grown in clay? Sure. But I'll bet the success rate is significantly lower as compared to those grown in sand at the same latitude & climate.

So, there are all kinds of things that come into play.

The major point I was trying to make was when we see an awesome looking plant online or wherever, and we see it rated to our zone and even colder, then we say, "Wait a minute. How come I don't know of a single one anywhere around where I live?", there may be very solid reasons for its absence. And those reasons may have nothing to do with cold.